Carotenoids in photosynthesis.

Wild—type photosynthetic organisms all contain carotenoids. These photosynthetic carotenoids are mainly packaged (together with the chlorophylls or bacteriochlorophylls) into specific pigment-protein complexes. This review lecture summarises the main types of photochemical reactions which carotenoids undergo in vitro and emphasises the organising role of the apoproteins, of these pigment—protein complexes, in controlling those reactions which are actually expressed in vivo. INTRODUCTORY REMARKS Carotenoids are essentially hydrophobic molecules and are typically found associated with photosynthetic membranes. However, they are not freely mobile within the liquid interior of these membranes, but are rather non—covalently bound to specific pigment—protein complexes. These complexes also usually contain chiorophylls or bacteriochlorophylls. In general carotenoids are rather reactive molecules being able to undergo a wide range of photochemical reactions. In vitro, in organic solvents, their photochemistry has been, and continues to be, extensively studied. In vivo, however, because the carotenoids involved in photosynthesis are bound into specific pigment—protein complexes, which of the possible photochemical reactions are expressed is largely determined by the various pigment-pigment and pigment—protein interactions that exist within these complexes. A detailed description of the structure of these pigment—protein complexes is therefore required if the behaviour of carotenoids in photosynthesis is to be clearly understood. At present, unfortunately, this information is largely unavailable. CAROTENO-CHLOROPHYLL (BACTERIOCHLOROPHYLL) -PROTEIN COMPLEXES In purple photosynthetic bacteria the carotenoids are found in association with two distinct classes of pigment-protein complexes, the reaction centres and the light— harvesting or antenna complexes (1,2). Both these types of complexes are hydrophobic, integral membrane proteins, which can be readily isolated and purified following detergent solubilisation. In Rhodopseudomonas sphaeroides, for example, two types of light-harvesting complexes are present (2—5) (the B875—complex and the B800—850—complex). The B875—complex exists in vivo as an aggregate of a minimal unit that contains two molecules of bacteriochlorophyll a and two molecules of carotenoid, bound to two low molecular weight apoproteins (the a—apoprotein Mr = 6809 and the 8—apoprotein Mr = 5441) (5,6). Both apoproteins have been sequenced (6). Similarly the B800—850—complex is an aggregate of three molecules of bacteriochlorophyll a, and 1 or 2 molecules of carotenoid, which are again bound to two low molecular weight apoproteins that have been sequenced (the a—apoprotein has an Mr = 5647, and the 8—apoprotein has an Mr = 5850) (4-9). By comparing the primary structures of a range of apoproteins from a variety of bacterial light harvesting complexes conserved histidine residues have been identified (one in the ct-apoproteins and two in the 6—apoproteins). It has been suggested that these conserved histidines may be ligands for the bacteriochlorophyll molecules (10). There is no clear evidence as yet as to where the carotenoids may be bound, however it has been suggested that this may occur near arginine,29, in the 8—apoprotein of the B800—850—complex in Rps. sphaeroides, since the buried charge associated with this arginine residue could account for the red—shift in the carotenoid's absorption spectrum when it is bound to the complex (6). 723 Unauthenticated Download Date | 7/9/17 5:17 AM